The invention relates to a circuit arrangement for demodulating an intermediate-frequency video signal, comprising a video demodulator and a phase-locked loop formed as a Costas loop, in which an output signal of a controllable oscillator arranged in the Costas loop and the intermediate frequency video signal are coupled to the video demodulator.
Circuit arrangements for demodulating IF video signals generally comprise a PLL for regaining the modulation frequency in the correct phase position. Conventional PLLs, all of which have a stable working point only every 360xc2x0, react to overmodulation of the amplitude-modulated television picture carrier with a phase jump of 180xc2x0. Due to this phase variation, the synchronous relation between the phase of the signal supplied by the PLL and the demodulator is subject to transient disturbances. Picture disturbances then occur.
It is further known that a Costas loop is used as a PLL for such demodulators, which loop is characterized in that it has stable working points at every 180xc2x0. When there is overmodulation in an IF video signal, the Costas loop can consequently lock in on the carrier phase then occurring in an inverted form, i.e. on the carrier phase of the picture carrier which has changed 180xc2x0 in phase. Thus, the above-described transient disturbances do not occur. However, another problem occurs, which is caused by the working points which are stable at every 180xc2x0. This problem is that, when the Costas loop locks in due to its stable working points at every 180xc2x0, a coincidental lock-in on one of two stable states at every 180xc2x0 is possible. When using a Costas loop in such a modulator, it is therefore not known in advance at which polarity the demodulated video signal occurs. Costas loops are known per se from the publication xe2x80x9cPhase-Lock Techniquesxe2x80x9d by Floyd M. Gardner, published by John Wiley and Sons. p. 221, particularly FIG. 11.6.
It is an object of the invention to provide a circuit arrangement of the type described in the opening paragraph with a Costas loop, which supplies the demodulated video signal always in a desired polarity.
According to the invention, this object is solved in that a polarity inverter is provided, which is preceded by the video demodulator and by means of which the polarity of the demodulated video signal is invertable in dependence upon a control signal supplied by a polarity detector, and in that the polarity detector determines the polarity of the output signal of the video demodulator or of the polarity inverter and, in dependence upon the determined polarity, controls the polarity inverter by means of the control signal in such a way that the demodulated video signal at the output of the polarity inverter has a predetermined polarity.
The circuit arrangement for demodulating an IF video signal comprises a polarity inverter. It is preceded by the video demodulator and allows inversion of the polarity of the demodulated video signal supplied by the video demodulator. Basically, a desired polarity of the demodulated video signal can be realized by means of this polarity inverter.
To this end, the polarity inverter is controlled by means of a control signal which is supplied by a polarity detector.
The polarity detector determines the polarity of the output signal of either the video demodulator or of the polarity inverter and controls the polarity inverter in dependence upon this determined polarity in such a way that the demodulated video signal at the output of the polarity inverter has a predetermined polarity.
A fundamental idea of the invention thus is to determine the actual polarity of the demodulated video signal and, in dependence upon this determined polarity, to control the polarity inverter in such a way that the demodulated video signal occurs with the desired polarity at the output of the circuit arrangement.
It is thereby ensured that the desired polarity occurs under all operating conditions of the circuit arrangement.
When there is overmodulation in the video signal and a resultant phase jump in the picture carrier at the output of the circuit arrangement, the demodulated video signal will neither be disturbed, nor will its polarity be inverted. Moreover, it makes no difference in the circuit arrangement according to the invention on which of the two 180xc2x0 phase-shifted working points the Costas loop locks in; the circuit arrangement will still always supply the video signal in the desired polarity.
An embodiment of the invention as defined in claim 2 has the particular advantage that a kind of feedback solution is possible when the polarity detector determines the polarity of the output signal of the polarity inverter (and not that of the output signal of the demodulator), in which solution the state of the control signal must be triggered only once in the polarity detector when the unwanted polarity of the demodulated video signal occurs, because the video signal will then occur in the correct, desired polarity at the output of the polarity inverter by virtue of the triggered change-over of this polarity inverter.
In accordance with a further embodiment as defined in claim 3, the circuit arrangement is advantageously formed in such a way that the demodulated video signal occurs with a negative polarity. In known circuit arrangements, the demodulated video signal is customarily processed in this polarity.
In accordance with a further embodiment of the invention as defined in claim 4, the above-described one-time reaction of the polarity detector upon occurrence of an unwanted polarity may advantageously be achieved by means of a comparator and a D-flipflop. When the value of the video signal applied to the comparator exceeds the value of a comparison signal, this comparator supplies a corresponding pulse to the D-flipflop. In response to this pulse, the D-flipflop changes its switching state and the control signal thus changes its state. Thereupon, the polarity inverter changes over and the demodulated video signal further occurs with the desired polarity after the polarity inverter. Consequently, the comparator subsequently does not trigger a switching pulse anymore.
The comparison voltage as defined in claim 5 advantageously has half the value of the voltage occurring in the video signal at maximum modulation. This applies both to positive and to negative polarities.
For multi-standard television receivers, in which it is generally determined or predetermined anyway which type of modulation is associated with the IF video signal, this modulation-type signal may advantageously also be used, as defined in claim 7, for the purpose of informing the polarity detector of the type of modulation and controlling the polarity inverter in a corresponding way. It can thereby be achieved that the circuit arrangement always has, for example, a demodulated video signal of a negative polarity, both in the case of negative modulation and in the case of positive modulation. This is a further essential advantage of the circuit arrangement according to the invention, because, in addition to the above-described advantages, it can also be used in this way to change the polarity of a television signal when it changes its polarity on the basis of positive or negative modulation. Also this polarity change can be suppressed by the circuit arrangement according to the invention.
For this purpose, the measures as defined in claim 8 are taken in a further embodiment of the invention. In accordance with the type of modulation of a video signal, the switchable inverter can invert or not invert the polarity of the demodulated video signal applied to the polarity detector. As a result, the polarity detector switches the polarity inverter always in such a way that the demodulated video signal appears in a fixed, desired polarity at its output, which is independent of the modulation type of the IF video signal.
Moreover, a DC position of the demodulated video signal, varying in accordance with the type of modulation, is compensated by means of the video amplifier.
In accordance with a further embodiment of the invention as defined in claim 9, there is an alternative possibility of changing the comparison voltage of the comparator in dependence upon the modulation type of the IF video signal, instead of changing the switchable inverter.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.